Pulse width modulation (PWM) is one of the dominating methods for control of electrical power supplied by a power supply to electronic means. Modern power transistors are capable of switching high voltages (≧1000 V) and high currents (≧100 A) with high switching speed. Pulse width modulation results in a very high efficiency, close to 100%.
Pulse width modulation has become a de facto standard especially for the supply of electric power to electric motors.
A three-phase bridge power supply for AC motor control is described as an example. The power supply may, however, be used also in other applications, for example in modern pulse width modulated audio amplifiers (often referred to as Class D amplifiers). Such amplifiers operate with a high PWM frequency, typically a few hundred kHz. They are provided with an LC filter at the output, with the effect that the operating conditions are similar to those in a single phase of a motor drive system. Audio amplifiers operate with a relatively low supply voltage and consequently voltage drops in the switch members may be detrimental.
A problem with all PWM power supply circuits is that the control accuracy may be influenced upon by voltage drops in different portions of the circuit.
One method for improving the control accuracy would be to measure the output voltage via an analog low-pass filter for the PWM-voltage provided at the output of the power supply and then make a feedback loop that controls the power supply and adjusts the output voltage to a correct value. This method is, however, sometimes not very useful, for example for motor control purposes. It would be necessary to have a low bandwidth in the analog filter, in order to get a sufficiently smooth waveform. This would make it necessary to have a low bandwidth also in the feedback loop and consequently it would be difficult to control high motor frequencies with good accuracy. Such a control method may work well for example in DC/DC-converters, but may work less well in motor control systems and other systems where the frequencies are typically 50 Hz and often much higher.
EP 0588213A2 discloses an inverter, which corrects an output voltage by the magnitude of output voltage error due to a dead time and an ON-state voltage drop in power elements to energize an AC motor. A motor current Iq is measured and the voltage drop is estimated as a function of the motor current from a function table or an approximation equation. The voltage drop is taken as the mean value of a voltage drop over the transistor and the voltage drop over the corresponding anti-parallel diode. In this way, a voltage detector is not required.
Another control approach is based on the geometry of PWM waveforms and does not require conventional feedback. The PWM waveforms from a modern frequency inverter are almost perfect square-waves. The mean value of such waveforms is a function of the pulse widths and the amplitudes. Consequently, it is sufficient to create PWM waveforms with correct amplitudes and pulse widths, which can be calculated or estimated, in order to obtain a correct mean value, which means that feedback may no longer be required.
However, the amplitudes of the PWM waveforms are distorted by unavoidable voltage drops in the power circuit. Thus, a new control strategy is desired, which takes advantage of the above-mentioned observations in order to provide improved control accuracy without conventional feedback. The strategy may as well be combined with any type of feedback.